Dynamic bone anchor

ABSTRACT

A dynamic bone anchor is provided comprising a longitudinal core member ( 2, 2′, 2″, 2 ′″) having a first end ( 21 ) and a second end ( 22 ) and a longitudinal axis (L) extending through the first end and the second end; at least one tubular segment ( 3, 3′, 3 ″) provided on the core member, the tubular segment having an outer bone engagement structure ( 31 ); wherein in a first configuration the at least one tubular segment is movable on the core member along the longitudinal axis; and wherein in an optional second configuration, the at least one tubular segment is fixed relative to the core member. Two or more tubular segments may be provided.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/746,367, filed on Dec. 27, 2012, in the U.S. Patent and Trademark Office, the entire contents of which is incorporated herein by reference; and claims priority from European Patent Application EP 12 199 487.5, filed Dec. 27, 2012, the entire contents of which is incorporated herein by reference.

BACKGROUND

1. Field

The invention relates to a dynamic bone anchor. The dynamic bone anchor comprises a longitudinal core member and a plurality of tubular segments provided on the core member. In a first configuration, there is a distance between the tubular segments in the longitudinal direction and the tubular segments are movable relative to one another. When the bone anchor is in the first configuration, the core member can perform small movements transverse to the longitudinal axis. In an optional second configuration, the tubular segments abut against each other and cannot move. The dynamic bone anchor can be used in any kind of bone fixation or stabilization device, such as pedicle screws or bone plates for the purpose of allowing a limited motion of components of the device.

2. Description of the Related Art

A dynamic bone fixation element is known from US 2009/0157123 A1. It includes a bone engaging component and a load carrier engaging component. The bone engaging component includes a plurality of threads for engaging a patient's bone and a lumen. The load carrier has a shaft portion that at least partially extends into the lumen. The distal end of the shaft portion is coupled to the lumen and at least a portion of an outer surface of the shaft portion is spaced away from at least a portion of an inner surface of the lumen via a gap so that the head portion can move with respect to the bone engaging component.

SUMMARY

It is the object of the invention to provide a dynamic bone anchor that allows a limited motion of a head of the bone anchor after anchoring the bone anchor into a bone or a vertebra. The object is solved by a dynamic bone anchor according to claim 1. Further developments are given in the dependent claims.

With the dynamic bone anchor, bone parts or vertebrae to be fixed or stabilized are able to can out a controlled limited motion relative to each other. In particular, the head of the bone anchor can perform a small rotational and/or translational motion with respect to the central axis of the bone anchor.

The bone anchor can assume in the assembled state a first configuration, in which the head is movable and optionally a second configuration, in which the whole bone anchor is a rigid device. The configuration of the dynamic bone anchor may be changed between the first configuration and the second configuration. In some embodiments, the second configuration may be used during insertion of the bone anchor into a bone part or vertebra. Because the bone anchor may be rigid during insertion, an easy insertion in a known manner is possible. The first configuration may be the configuration of the implanted bone anchor.

A bone engagement structure of the dynamic bone anchor may be a bone thread or another engagement structure such as barbs or the like. If the bone engagement structure is a bone thread, the thread portions associated with the tubular segments of the bone anchor are orientated with respect to each other such that in the second configuration a continuous bone thread is provided along the outer surface of the bone anchor. The orientation is not changed when the bone anchor assumes the first configuration. This opens the possibility of performing corrections of the position of the bone anchor after implantation by reverting to the second configuration.

The dynamic bone anchor may comprise a head that can assume various designs. in particular, a spherical head can be used that renders the dynamic bone anchor suitable for combination with receiving parts of polyaxial bone screws or with bone plates comprising holes with spherical seats for pivotably accommodating the bone screws.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of the dynamic bone anchor according to a first embodiment;

FIG. 2 shows a perspective view of the bone anchor according to FIG. 1 in an assembled state;

FIG. 3 shows side view of the core member of the dynamic bone anchor according to FIG. 1.

FIG. 4 shows a bottom side view of the tip of the core member according to FIG. 3;

FIG. 5 shows a perspective view of a tip member of the dynamic bone anchor according to the first embodiment;

FIG. 6 shows a cross-sectional view of the tip member shown in FIG. 5, the cross-section taken in a plane containing the longitudinal axis of the core member;

FIG. 7 shows a perspective from the bottom of a head of the dynamic bone anchor according to FIG. 1;

FIG. 8 shows a cross-sectional view of the head of FIG. 7, the cross-section taken in a plane containing the longitudinal axis of the core member;

FIG. 9 shows a side view of the tubular segments of the dynamic bone anchor in the first configuration according to the first embodiment as shown in FIG. 1;

FIG. 10 shows a cross-sectional view along line A-A in FIG. 9;

FIG. 11 shows a side view of the tubular members of FIG. 9 in the second configuration in which the tubular segments abut against each other;

FIG. 12 shows a cross-sectional view of the dynamic bone anchor of FIGS. 1 and 2 in the second configuration, wherein the cross-section is taken in a plane containing the longitudinal axis;

FIG. 13 a shows a cross-sectional view of the dynamic bone anchor according to the first embodiment in the second configuration in which the tubular members are spaced apart from each other by a small distance;

FIG. 13 b shows an enlarged view of a detail of FIG. 13 a;

FIG. 14 shows a cross-sectional view of a polyaxial pedicle screw wherein the dynamic bone anchor according to the first embodiment is used as an anchoring element;

FIG. 15 shows a cross-sectional view of the dynamic bone anchor according to the first embodiment used with a bone plate;

FIG. 16 shows a perspective exploded view of a dynamic bone anchor according to a second embodiment;

FIG. 17 shows a perspective exploded view of the dynamic bone anchor according to the second embodiment in an assembled state;

FIG. 18 shows a cross-sectional view of the dynamic bone anchor according to the second embodiment in the second configuration, the cross-section taken in a plane containing the longitudinal axis;

FIG. 19 shows a cross-sectional view of the dynamic bone anchor of FIG. 18 in the first configuration in which the tubular elements are spaced apart from each other by a small distance;

FIG. 20 shows a perspective exploded view of a dynamic bone anchor according to a third embodiment;

FIG. 21 shows a perspective view of the dynamic bone anchor of FIG. 20 in an assembled state;

FIG. 22 shows a perspective view from the bottom onto the head of the dynamic bone anchor according to the third embodiment;

FIG. 23 shows a cross-sectional view of the dynamic hone anchor according to the third embodiment in the second configuration, wherein the section is taken in a plane containing the longitudinal axis;

FIG. 24 a shows a cross-sectional view of the dynamic bone anchor of FIG. 23 in the first configuration in which the tubular segments are spaced apart by a small distance;

FIG. 24 b shows an enlarged view of a detail of FIG. 24 a;

FIG. 25 shows a perspective exploded view of a dynamic bone anchor according to a fourth embodiment;

FIG. 26 shows a perspective view of the dynamic bone anchor according to FIG. 25 in an assembled state;

FIG. 27 shows a perspective view from the bottom onto the head of the dynamic bone anchor according to the fourth embodiment;

FIG. 28 a shows a cross-sectional view of the dynamic bone anchor according to the fourth embodiment in the first configuration wherein the tubular segments are spaced apart from each other by a small distance, wherein the section has been taken in a plane containing the longitudinal axis; and

FIG. 28 b shows an enlarged view of a detail of FIG. 28 a.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, there is shown a dynamic bone anchor 1 according to a first embodiment that includes a core member 2, a plurality of tubular segments 3, 3, 3″, a tip member 4 and a head 5. The tubular segments 3, 3′, 3″ can be placed onto the core member 2 and the tip member 4 and the head 5 can be connected to the core member 2 to form the bone anchor 1. Referring further to FIG. 3, the core member 2 comprises a first end 21, an opposite second end 22 and a longitudinal axis L extending through the first end 21 and the second end 22 and forming a central axis of the bone anchor when the core member is not bent or deflected.

The core member 2 can have a tip at the first end 21 and has a connection structure 23 adjacent to the tip 21, which can be a thread as shown in. FIG. 3, A central portion 24 between the connection structure 23 and the first end 22 is rod-shaped and has a circular cross-section. At a distance from the connection structure 23 a plurality of connection portions 25 are provided along the length of the central portion 24, wherein the connection portions 25 are spaced apart from each other equidistantly. The connection portions 25 have a square-shaped outer contour as can be seen in FIGS. 1 and 4 and have a thickness so that they extend beyond the outer surface of the rod-shaped portion 24 in a radial direction. In the embodiment shown, three connection portions 25 are provided that serve for connecting the three tubular segments 3, 3′, 3″ shown in FIG. 1 to the core member 2. At a distance from the last connection portion 25 towards the second 22, a further connection portion 26 is provided that serves for connecting the head 5 to the core member 2 as explained below. The connection portion 26 also has a square-shaped outer contour, Adjacent to the connection portion 26 for the head 5 in a direction towards the second end 22, a cylindrical portion 27 is provided that has an external diameter greater than the diameter of the rod-shaped portion 24 and that is accommodated in the head 5. Between the cylindrical portion 27 and the second end 22, a predetermined break-off section 28 is provided. The predetermined break-off section 28 is realized by a region with a reduced outer diameter of the rod-shaped portion 24. The predetermined break-off section 28 serves for adjusting the length of the rod-shaped portion 24 after implantation of the bone anchor into the bone.

The tip member 4 is a segment of a cone with a threaded axial hole 41 configured to cooperate with the connection structure 23 as shown in FIGS. 1 and 5 to 6. The tip member 4 and the tip portion at the first end 21 form the tip of the bone anchor. When the tip member 4 is screwed onto the core member 2, it abuts against the rod-shaped portion 24 and extends beyond the rod shaped portion 24 in a radial direction as can be seen in particular in FIG. 12. Thereby, a first annular stop surface 42 is provided for the tubular segments 3, 3′, 3″.

The head 5 will be explained with reference to FIGS. 7 and 8. The head 5 comprises a first end 51, an opposite second end 52 and a spherical segment-shaped portion 53 adjacent to the first end 51. Between spherical-segment shaped portion 53 and the second end 52, a short neck portion 54 is present that has a substantially cylindrical shape. A recess 55 with a square-shaped inner contour extends from the second end 52 into the spherical-segment portion 53. The recess 55 serves for accommodating the connection portion 26 of the core member 2 and is adapted to provide a form-fit connection between the connection portion 26 and the head 5.

A tubular shaft of the bone anchor is divided into a plurality of tubular segments 3, 3′, 3″, wherein a first end segment 3 adjacent to the tip member 4, a second end segment 3′ adjacent to the head 5 and one or more intermediate tubular segments 3″ are provided. Each of the tubular segments 3, 3′, 3″ comprises a bone thread 31 at its outer surface. When the tubular segments 3, 3′, 3″ abut against each other, the bone thread on each tubular member fits to the bone thread of the abutting tubular member, so that a continuous tubular shaft with a continuous bone thread is formed. The first end member 3 can have a tapering portion 32 that tapers towards the first end 21 of the core member when the tubular segments are placed onto the core member 2. The second end tubular member 3′ may comprise a cylindrical portion 33 that is directed to the second end 22 of the core member 2. The cylindrical portion 33 and the tapered portion 32 are threadless. It should be understood that the bone thread 31 can be any suitable bone thread. It is not necessary that the bone thread extends fully over each tubular segment nor is it necessary that the bone thread is present on all tubular segments.

The inside 34 of the tubular segments 3, 3′, 3′ hollow and may have a contour in a plane perpendicular to the longitudinal axis that is adapted to the shape of the connection portions 25 of the core member 2. In the embodiment shown, the contour is square-shaped. However, corresponding to the shape of the connection portions, any polygonal or otherwise formed shape is possible that prevents rotation of the tubular segments 3, 3′, 3″ around the longitudinal axis when the tubular segments are placed onto the connection portions 25.

The length of the tubular segments is such that when the tubular segments are placed onto the core member 2 and abut against the tip member 4, there is a distance between the connection portion 26 and the free end of the second end segment 3′.

The material of the core member 2, the tubular segments 3, 3′, 3″, the tip member 4 and the head 3 is preferably a body-compatible material, such as body-compatible metal, for example titanium or stainless steel, a body-compatible metal alloy, such as for example a nickel titanium alloy, in particular Nitinol, or a body-compatible polymer material, for example polyetheretherketone (PEEK). The parts can be all of the same or of different materials.

The dynamic bone anchor 1 is assembled as follows. The core member is guided through the recesses 56, 55 with the first end 21 until the cylindrical portion 27 is accommodated in the recess 56 of the head 5 and abuts against the bottom 56 a of the recess 56. Then, the tubular segments 3, 3, 3″ are placed onto the core member 2 such that each tubular segment is positioned on a connection portion 25. Thereafter, the tip member 4 is mounted onto the core member 2. A portion adjacent to the second end 22 of the core member 2 acts a traction portion.

In the assembled state, the dynamic bone anchor 1 can assume a first configuration in which the tubular segments 3, 3′, 31″ rotationally fixed on the core member 2 but are slideable in axial direction in a limited manner because there is a small distance between them. In the first configuration, because the tubular segments 3, 3′, 3″ can move to a limited extent in axial direction, the core member 2 with the head 5 is able to move relative to the tubular segments away from the longitudinal axis L.

The dynamic bone anchor can assume a second configuration, in which the end surface 52 of the head abuts against the free end of the second end member 3′ and shifts the tubular segments towards the stop 42 at the tip member 4. In the second configuration, the tubular segments are not movable in an axial direction. As in the first configuration, the tubular segments 3, 3′, 3″ are also not rotatable.

In use, the core member 2 is engaged by a tool (not shown) and drawn away from the end surface 51 of the head 5, while the head 5 may serve as abutment for the tool. By means of this, the head 5 presses onto the second end segment 3′ as shown in FIG. 12 and shifts all the tubular segments 3, 3′, 3″ towards the tip member 4. The distance between the tubular segments 3, 3′, 3″ is eliminated and the whole bone anchor has a continuous bone thread on its outer surface. In this second configuration, the head 5 and the tubular segments 3, 3′, 3″ are pre-tensioned. The bone anchor is rigid, i.e. there is no relative movement of the parts. The bone anchor can be inserted into a bone part or a vertebra. The insertion force is transmitted via the connection portions 25 onto the tubular segments. Because the tubular segments 3, 3′, 3″ cannot rotate, the orientation of the thread portions with respect to each other is maintained.

After insertion, the core 2 is released by the tool so that the pre-tension is not maintained. The head 5 can move slightly in an axial direction so that the cylindrical portion 27 abuts against the abutment 56 a in the head 5. As a result, small gaps emerge between the tubular segments 3, 3, 3″ that allow a limited motion of the tubular segments relative to each other. As shown in FIG. 13 a, the head with the core can perform a small translational and/or rotational movement with respect to the tubular segments 3, 3′, 3″ in a direction transverse to the longitudinal axis L. Such a movement is based on a deflection of the core member away from the straight position which is possible due to a gap 26 a between the rod-shaped portion 24 of the core member 2 and the tubular segments (FIG. 13 b). As can be further seen in FIG. 13 b, because the tubular segments are movable to a limited extent, the end surface 52 of the head is movable with respect to the tubular segment 3.

Finally, the core can. be shortened by breaking-off the second end 22 at the predetermined break-off section 28.

A first application of the bone anchor together with a stabilization device is shown in FIG. 14. The bone anchor according to the first embodiment is coupled to a receiving part 60 to form a polyaxial bone anchor. The receiving part 60 is substantially cylindrical and comprises a top end 61, a bottom end 62 and a coaxial bore 63 extending from that top end 61 to a distance from the bottom end 62, The bore 63 narrows towards the bottom end 62 and provides an opening 64 near the bottom end 62. Near the opening 64, a seat 65 is provided for pivotably receiving the head 5. AU-shaped recess extends from the top end 61 to a distance from the top end 61 for receiving a stabilization rod 70. By means of the U-shaped recess two free legs 66, 67 are provided, which have an internal thread 68 for cooperating with a locking member such as a set screw 80. Furthermore, a pressure member 90 is provided that exerts pressure onto the head 5, such that the head 5 can be locked in a certain angular position by tightening the locking member 80. The bone anchor may be used with other designs of receiving parts and polyaxial bone screws. Also the head 5 of the core member 2 may be designed such that it comprises a section for receiving a rod and for receiving a locking member to fix the rod as known from other monoaxial bone screws.

In use, at least two polyaxial bone anchors are inserted into adjacent vertebrae or bone parts and connected via the rod 70. Once the bone anchors 1 are inserted into the bone parts or adjacent vertebrae, the heads 5 can perform a limited motion with respect to the tubular segments. Once a head 5 is locked in the receiving part 60, the bone anchor provides for a dynamic stabilization that allows small movements of the bone parts with respect o each other or small movements of a motion segment of the spinal column.

A second example of an application is shown in FIG. 15, wherein bone anchors 1 according to the first embodiment are used together with a bone plate 100 comprising holes 100 a with seat portions 100 b for receiving the heads 5 of the two bone anchors, respectively. The two bone anchors are inserted in adjacent bone parts and the bone plate 100 bridges at least a portion of a fracture site. In a specific application, a distance between the center axis of the two holes 100 a is slightly smaller than a distance between the longitudinal axis L of the bone anchors 1. Because the heads 5 with the core members 2 can move slightly in a direction transverse to the longitudinal axis, the bone parts can he drawn together at the fracture site.

A second embodiment of the dynamic hone anchor will be explained with reference to FIGS. 16 to 19. The dynamic bone anchor 1′ of the second embodiment differs from the dynamic bone anchor 1 of the first embodiment in the design of the core member and the head. Parts that are identical or similar to the first embodiment, are indicated with the same reference numerals and the description thereof will not he repeated. The core member 2′ comprises adjacent to its second end 22 a cylindrical portion 29 with an engagement portion 29 a for a tool, for example a slot 29 a. Following the cylindrical portion 29, there is a threaded portion 26′ with an outer thread that is configured to cooperate with a corresponding thread provided in the head. The head 5′ shown in

FIGS. 18 and 19, differs from the head 5 in that the recess 55′ that serves for connection to the core member 2′ is circular and has an internal thread, that cooperates with the threaded connection portion 26′ of the core member 2,

In the assembled state shown in FIGS. 17 to 19, the head 5′ is screwed onto the core member 2′ and presses with its free end surface 52 onto the free end surface of the second end member 3′. By means of the threaded connection between the core member 2′ and the head 5′, the tubular segments can be pre-tensioned with respect to the core member 2′ through pressing them against the tip member 4. In this second configuration of the dynamic bone anchor, which is shown in FIG. 18, the dynamic bone anchor is rigid. The dynamic bone anchor can be inserted into the bone in the second configuration. For example, the slot 29 a can be engaged with a tool and the whole bone anchor can be screwed into the bone.

After insertion into the bone, the core member 2′ can he screwed backward relative to the head 5′, until the cylindrical portion 29 abuts against the abutment 56 a in the recess 56 as shown in FIG. 19 which depicts the first configuration of the bone anchor. Thereby, the pre-tension is released and the tubular segments are movable in an axial direction to a limited extent.

A. third embodiment of the dynamic bone anchor will be described with reference to FIGS. 20 to 24 b. The third embodiment 1″ of the bone anchor differs from the previous embodiments by the design of the core member 2″ and the head 5″. All other parts are similar or identical to the previous embodiments and the description thereof will not be repeated. The core member 2″ comprises a connection portion 26″ for connection to the head 5″. The connection portion 26″ has an outer polygon contour, in particular a square-shaped contour that is configured to be connected to the corresponding square-shaped recess 55 of the head 5″ by a press-fit connection. At the opposite side, the head 5″ has an engagement portion 58 for a tool, for example, a torx-shaped recess 58.

The core member 2″ is made of a material that is based on a nickel-titanium based shape memory alloy, preferably from Nitinol. The material exhibits shape memory properties.

The core member 2″ is connected to the head 5″ via a press-fit connection achieved through the shape memory effect. For example, the connection portion 26″ is cooled below the martensite finish temperature M_(t) so that the flat sides of the connection portion 2611 are impressed. Due to the ability of the martensite phase to deform, the connection portion 26″ can be easily inserted into the recess 55 and after heating can return to the square shape to achieve the press-fit connection.

To provide the first and the second configuration of the bone anchor, also the shape memory effect is used. First, the core 2″ can be cooled below the martensite finish temperature M_(r) before mounting and can be slightly compressed in the longitudinal direction, whereby its length is shortened. In the martensite metallurgical state, the core member 2″ with the head 5″ is assembled with the tubular segments 3, 3′, 3″ and the tip member 4. After heating above the austenite finish temperature A_(f), the core member 2″ assumes its original non-compressed state and its original length. The prolongation of the core member 2″ after heating moves the head 5″ away from the tubular segments so that the tubular segments become movable with respect to each other in an axial direction.

The heating can be carried out by the application of body temperature, for example, when the bone anchor is inserted into a bone or can be carried out through a separate heating step with an external heating device.

A fourth embodiment of the dynamic bone anchor will be described with reference to FIGS. 25 to 28 b. The fourth embodiment the dynamic bone anchor 1 differs from the previous embodiments in the design of the core member 2″ and the head 5″'. Parts that are similar or identical to those of the previous embodiments have the same reference numerals and the description thereof will not be repeated. The core member 2′ comprises adjacent to the engagement portion 23 the form of a threaded portion first connection portion 25′ that has a square-shaped outer contour as the other connection portions 25′ and provides for a greater abutment surface for the tip member 4. The other connection portions 25′ are provided at positions P1, P2, etc. that are corresponding to positions at which the ends of the tubular segments abut against each other as shown in detail in FIG. 28 a. Hence, the tubular segments 3, 3′, 3″ are supported by the connection portions 25 at their respective free ends. Adjacent to its second end 22, the core member 2″ comprises a connection portion 26′ with a square-shaped outer contour as in the third embodiment. The head 5″ is similar to the head of the first embodiment and has a square-shaped recess 55 for connection with the connection portion 261′″ and opposite thereto a recess 58 for engagement with a tool. The connection portion 26′ extends in a radial direction beyond the connection portion 25′. Furthermore the axial length of the connection portion 26′ may be slightly shorter than the depth of the recess 55 in the head 5″ so that a small gap 27 a may be provided within the neck portion of the head 5″ that further facilitates movement of the head 5″ in the first configuration. The connection portion 26″ can be connected to the head 5′″, for example, by a press-fit connection,

As in the previous embodiments, a configuration, show in FIGS. 28 a and 28 b is provided in which the tubular segments 3, 3′, 3″ are movable with respect to each other and wherein the head 5′″ can perform a limited motion with respect to the tubular segments. The dynamic bone anchor can be inserted in this configuration because higher forces can be transmitted via the abutting end surfaces of the tubular segments 3, 3′, 3″,

It should be understood that also for the fourth embodiment the core member 2′″ can be made of a material based on a NiTi-shape memory alloy, such as Nitinol. Also in this embodiment, the core member 2′″ may be shortened by compressing it in the martensite phase and can be prolonged by heating it after insertion of the bone anchor into the bone. Therefore, it is possible to obtain two dynamic configurations with different sizes of the gaps,

Further adaptations or modifications of the dynamic bone anchor described in the embodiments can be accomplished by one of ordinary skill in the art without departing from the scope of the invention. For example, the head may have any other shape suitable for connecting it to other stabilization devices such as bone plates, receiving parts for accommodating stabilization rods, etc. The head may even be omitted in some embodiments, if the free end of the core member is suitable for connection to another device.

Any kinds of tips may be provided. The tip member 4, for example, can be full a cone and the first end 21 can be only provided with a connection structure to connect to the tip member.

For the bone engagement structure, barbs or any other bone engagement structure may be provided, for example, a roughened surface.

At least one tubular segment, preferably two ore more tubular segments may be provided.

The features of different embodiments may also be combined with each other.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. 

1. Dynamic bone anchor comprising a longitudinal core member having a first end and a second end and a longitudinal axis extending through the first end and the second end; at least one tubular segment provided on the core member, the tubular segment having an outer bone engagement structure; wherein in a first configuration the at least one tubular segment is movable on the core member along the longitudinal axis.
 2. The dynamic bone anchor of claim 1, wherein in the first configuration the core member is movable relative to the at least one tubular segment in a direction transverse to the longitudinal axis.
 3. The dynamic hone anchor of claim 1, wherein in the first configuration, the at least one tubular segment is configured to slide on the core member but is prevented from rotation around the longitudinal axis.
 4. The dynamic bone anchor of claim 1, wherein an inner surface of the at least one tubular segment comprises in a plane perpendicular to the longitudinal axis a polygon-shaped contour, preferably a square-shaped contour, and wherein the core member comprises connections portions at positions corresponding to the at least one tubular segment with a corresponding outer contour.
 5. The dynamic bone anchor of claim
 1. wherein the at least one tubular segment comprises a bone thread on its outer surface.
 6. The dynamic bone anchor of claim
 1. wherein a first stop and a second stop is provided on the core member that limits the motion of the at least one tubular segment along the longitudinal axis in the first configuration.
 7. The dynamic bone anchor of claim 6, wherein the first stop is formed by a tip member provided at or near the first end of the core member, preferably by a connectable tip member.
 8. The dynamic bone anchor of claim 6, wherein the second stop is formed by a head provided at or near a second end of the core member.
 9. The dynamic bone anchor of claim 8, wherein the head is slidable on the core member so that it can move against the at least one tubular segment.
 10. The dynamic bone anchor of claim 9, wherein the core member comprises a traction portion at its second end and wherein the bone anchor is brought into a second configuration by pulling the core member and pressing the head against the at least one tubular segment.
 11. The dynamic bone anchor of claim 1, wherein the core member has a predetermined break-off section at a distance from its second end.
 12. The dynamic bone anchor of claim 8, wherein the head is configured to be connected to the core member at an adjustable distance to at least one tubular segment in a longitudinal direction, preferably by a threaded connection.
 13. The dynamic bone anchor of claim 1, wherein the core member is made of a material comprising shape memory properties and wherein the core member is configured to assume a first length in the first configuration and a second length in a second configuration and wherein the first length is greater than the second length.
 14. The dynamic bone anchor of claim 13, wherein the core member is configured to change from the second length to the first length upon the application of heat.
 15. The dynamic bone anchor of claim 1, wherein at least two tubular segments are provided and wherein the core member comprises at least one connection portion at a position to bridge the ends of the tubular segments that are facing each other.
 16. The dynamic bone anchor of claim 1, wherein the dynamic bone anchor can assume a second configuration in which the at least one tubular segment is fixed with respect to the core member. 17-20. (canceled) 